A single-stranded filamentous vector for cloning DNA has been developed. It will be used to clone fragments of DNA from the nematode Caenorhabditis elegans. These cloned fragments will be used as probes for an extensive, sensitive search for developmentally programmed small rearrangements in the genome of the nematode cells. They will also be used to "stain" individual cells for complementary RNA sequences, with the idea ultimately of obtaining a complete catalogue of what cells in the worm contain transcripts of what individual genes.